The present invention relates to diamond workpieces and more particularly to their fabrication based on chemical vapor deposition technology.
Its hardness and thermal properties are but two of the characteristics that make diamond useful in a variety of industrial components. Initially, natural diamond was used in a variety of abrasive applications. With the ability to synthesize diamond by high pressure/high temperature (HP/HT) techniques utilizing a catalyst/sintering aid under conditions where diamond is the thermally stable carbon phase, a variety of additional products found favor in the marketplace. Polycrystalline diamond compacts, often supported on a tungsten carbide support in cylindrical or annular form, extended the product line for diamond additionally. However, the requirement of high pressure and high temperature has been a limitation, for example, in product configuration.
Recently, industrial effort directed toward the growth of diamond at low pressures, where it is metastable, has increased dramatically. Although the ability to produce diamond by low-pressure synthesis techniques has been known, drawbacks including extremely low growth rates prevented wide commercial acceptance. Recent developments have led to higher growth rates, thus spurring recent industrial interest in the field. Additionally, the discovery of an entirely new class of solids, known as "diamondlike" carbons and hydrocarbons, is an outgrowth of such recent work.
Low pressure growth of diamond has been dubbed "chemical vapor deposition" or "CVD" in the field. Two predominant CVD techniques have found favor in the literature. One of these techniques involves the use of a dilute mixture of hydrocarbon gas (typically methane) and hydrogen wherein the hydrocarbon content usually is varied from about 0.1% to 2.5% of the total volumetric flow. The gas is introduced via a quartz tube located just above a hot tungsten filament which is electrically heated to a temperature ranging from between about 1750.degree. and 2400.degree. C. The gas mixture disassociates at the filament surface and diamonds are condensed onto a heated substrate placed just below the hot tungsten filament. The substrate is held in a resistance-heated boat (often molybdenum) and heated to a temperature in the region of about 500.degree. to 1100.degree. C.
The second technique involves the imposition of a plasma discharge to the foregoing filament process. The plasma discharge serves to increase the nucleation density, growth rate, and it is believed to enhance formation of diamond films as opposed to discrete diamond particles. Of the plasma systems that have been utilized in this area, there are three basic systems. One is a microwave plasma system, the second is an RF (inductively or capacitively coupled) plasma system, and the third is a d.c. plasma system. The RF and microwave plasma systems utilize relatively complex and expensive equipment which usually requires complex tuning or matching networks to electrically couple electrical energy to the generated plasma. Additionally, the diamond growth rate offered by these two systems can be quite modest.
Symmetrical diamond coating of an object with the hot filament CVD diamond technique can be accomplished by rotating the object about an axis parallel to the filament during deposition. However, substrate rotation can be a source of other problems. Rotating the substrate has been found to generate periodic temperature fluctuations of the surface of the object as it passes from its own day-to-night-to-day cycle, often causing the coating to fracture. Such a method is disclosed in Japanese Application No. 62-296707.
In a related process, diamond is deposited on a series of stationary substrate wires positioned around a number of filaments to enhance uniform growth on the wire. After deposition of the diamond, the coated wire is removed from the furnace and the metal is acid leached away in order to leave a diamond tube. The tubes formed have found use in water-jet cutting technology and may be useful as orifices, mixing tubes and nozzles and, in addition, they can be used as wire dies or guides.
While this technique provides diamond tubes without fracture, these procedures do not always provide uniform wall thicknesses. Techniques are desired which will provide improved uniformity in the wall thickness with uniform inner diameters.